This invention relates to integrated circuit lithography and more particularly to the use of various properties of light in making the integrated circuits.
A technique that is becoming important in the drawing of lines and features in an integrated circuit is a technique known as phase shift, which refers to a type of mask which provides for a 180 degree phase shift between various portions on the mask. That is, the light passing through the mask in some areas has a 180 degree phase shift in relation to light passing to the mask in other areas. This technique is very useful in drawing sharp lines by providing cancellation of the light at these phase shift boundaries where the light has the 180 degrees phase difference. One of the problems relating to this technology is that this 180 degrees phase difference also occurs in areas where lines are not necessarily intended to be drawn. The result has been that in these regions where there is no line intended to be drawn a second mask is utilized to remove or ensure exposure of the photoresist in those regions. In addition to the extra cost of this extra step, there are also alignment issues between the two mask steps. One of the difficulties with the use of phase shift masks has been the problems associated with the areas of interface between the different phase shifted areas. This difficulty has impeded the development of phase shift masks in the processing of integrated circuits.
One of the difficulties that arises is that the constraints on how the different features can be arranged and the typical routing techniques for maximizing the density of the packing of the different features has to be altered. Thus, this requirement of the second mask reduces the efficiency of the use of the available space that is available and creates additional concerns that must be accounted for in checking for violations of layout rules. Further, by virtue of having to utilize the second mask, there are alignment issues that create marginality problems that can result in the circuit not working as designed or the circuit designer having to take into account that these margin variations will occur in the operation. Furthermore, the two differing areas of phase shift limit the geometrical patterns that can be actually patterned in this manner.
Thus, there is not the full generality of layout possibilities that is present normally, without the use of phase shift masks. Thus there is seen a need for an improved ability to use phase shift masks in achieving circuit designs.